This invention relates to a rate adaptive pacemaker and, more particularly, to a pacemaker employing the Tx principle of controlling rate as a function of sensed stimulus-T wave interval.
A physiologically adaptive cardiac pacemaker, wherein the desired pacing rate is determined as a function of sensed stimulus-T wave (Q-T) interval has been shown in U.S. Pat. No. 4,228,803, Rickards. This pacemaker, which is now being produced commercially, is referred to as the Tx rate adaptive pacemaker. The use of a microprocessor, or equivalent programmable circuitry in a pacemaker, such as shown in EPO Application No. 81108940.8, makes the Tx pacer more readily feasible. Other improvements to this principle are disclosed in U.S. Pat. No. 4,305,396, Wittkampf et al.
The Tx rate adaptive pacer makes possible an improvement in determining the refractory interval for a ventricular (or atrial) pacer which has not been heretofore available. In conventional pacemakers, it is necessary to set the refractory interval safely after the expected time of occurrence of the T wave. However, this safety factor is at the expense of QRS sensing, since a longer refractory period results in a shorter sense period. In the Tx pacer, however, the occurrence of the T wave is sensed, and accordingly information is readily available for terminating the refractory period directly after the sensed T wave.
A normal premise of a rate adaptive demand pacemaker, Tx or otherwise, is that when the natural patient heart rate is being sensed and the pacemaker stimulus delivery is being inhibited, an optimum condition obtains. In other words, when the heart is able to operate without intervention of the pacemaker, it should do so. However, at the same time, there may be conditions when the pacemaker should overtake or overdrive the natural rate. For the Tx pacer, as long as pacing is inhibited, no Q-T data is acquired from which a pacing rate can be determined. Under such circumstances, it is desirable to get the Q-T rate indication and adjust the pacing rate, either to overtake the heart or to have the proper pacing rate established in the event of loss of the natural beat.
Another need of adaptive rate pacers generally is that of periodically checking and adjusting the correlation between a pacer indicated rate and the sensed physiological parameter, or data. For example, for the Tx pacer, a correlation or sensitivity function in terms of beats per minute (bpm) per Q-T interval (ms) must be programmed into the microprocessor; it is desirable to be able to check that correlation function and determine, at any point in the lifetime of the implanted pacemaker, whether the correlation function is appropriate, or whether it should be adjusted in view of data concerning the patient.